1. Field of the Invention
This invention relates to coating metal surfaces and more particularly to the coating of metal surfaces with solutions.
2. Description of the Prior Art
The prior art is replete with processes and solutions for performing chromate/fluoride conversion coatings on metal surfaces to enhance corrosion resistance and to promote paint adhesion. Typical of processes of this type are those described in United States patents Nos. 2,276,353, 2,471,909, 2,472,864, 2,796,370, 2,796,371, 2,507,966, 2,839,439, 2,843,513, 2,859,144, 2,868,679, 3,009,842. In spite of these advances problems remain unsolved while, at the same time, new problems due to advancing technology are created. For example, in a coil coating process for coating aluminum, commercial high speed lines are now in operation whereby it is possible to continuously process strips of metal at speeds of more than 1000 square feet per minute. Increasing the line speed promotes efficiency of the operation yet presents certain difficulties where chromate conversion coating solutions are employed because of the necessity of allowing a finite time period for the contacting of the metal surface with the solution. In an attempt to increase the rate of the chromate conversion coating step, additions of activating agents, known as bath accelerators, are sometimes made to the bath solutions. The purpose of adding the activating ions relates to their ability to increase the coating weight in a given period of time, e.g., coating efficiency of the bath. The result is a better final product. One activating substance which has met with some success is ferricyanide. See for example, U.S. Pat. Nos. 2,796,370, 2,796,371 and 2,988,465.
Although the ferricyanide activated chromate coating solutions have enjoyed appreciable commercial use, there are problems associated with their utility. For example, it has been found that the ferricyanide activated solution is subject to heat degradation at higher temperatures and also is relatively sensitive to changes in acidity. However, perhaps the most serious obstacle to the use of ferricyanide today relates to the disposal problem of the spent solutions because of the sometimes occurrence to free cyanide moieties.
At least one non-ferricyanide process for aluminum is known (U.S. Pat. No. 3,391,031) but this process requires the use of tungstanate material.
Today, as never before, there is an urgent need to develop coating compositions and methods for their use which are not only economical but will also result in a metal surface with enhanced corrosion and paint bonding characteristics - and which will achieve these benefits without detrimental environmental impact.